1. Field of the Invention
This invention pertains to a subrate exchange system for use in a digital communications network, and more particularly to a subrate control channel exchange system using some bits in time slots of control channels for transmitting a control signal and the remaining bits for transmitting a voice signal.
2. Description of the Related Art
A time-divisional digital exchange generally exchanges data in 8-bit units. Yet, there are cases in which not all the bits in such data are significant. There is a need, in those cases, for improving an efficiency in line utilization by sending and receiving only significant data in 8-bit units by multiplexing the exchange of individual bits, i.e. a subrate exchange, by an exchange.
Recent events, such as digitization of public networks as an ISDN, the advent of intracorporate high speed digital communications networks, and the prevalence of various data terminals (e.g. personal computers), cause increases not only in voice communications by telephone sets but also in data communications amongst data terminals or between data terminals and host computers.
This necessitates an intercorporate and intercorporate transmission line network to evolve from a conventional analog transmission line network into a digital transmission line network to be further built as a private ISDN network from the standpoint of service improvement and future enhancement.
FIG. 1 shows an exemplary configuration of an intracorporate communications network. Ordinarily, an ISDN primary rate interface (ISDN-PRI), as in a so-called 23B+D format, uses, as the D channel for transmitting control signals, one [1] of the twenty-four [24] channels as line numbers of transmission paths and the remaining twenty-three [23] channels, for example, for transmitting voice signals. These twenty-four [24] channels have a transmission speed of sixty-four kilobits per second [64 kbps].
For instance, in the example shown in FIG. 1, although data are transmitted between a head office 90 and branch offices 91, over main lines 92, by fully taking advantage of all 23 B channels and a D channel, there are few instances in which such high speed communications are required between liaison 93 offices and sub-branch offices 94, where low speed data transfers approximating personal computer communications would accomplish the job. Therefore, local lines 96 are used.
That is, although one [1] line has hitherto allowed one [1] sequence of data communications when communications are modulated into analog modem signals for exchanging data, a digital communications path, especially an ISDN, has come to ensure a 64 kbps data communications throughput, and the demand for a high-speed data communications such as 48 kbps, 58 kbps, 64 kbps, etc. has been satisfied. Nevertheless, there are cases in which low speed data transfers far below 64 kbps, e.g. 1.2 kbps, 2.4 kbps, 4.8 kbps, 9.6 kbps and 19.2 kbps, are required, where a data transfer pursuant to the CCITT advised v.110 format is needed.
In those cases, an 8-bit transfer carries only one [1] bit, two [2] bits or four [4] bits of significant data, thereby lowering the line efficiency.
FIGS. 2 and 3 conceptually show a conventional exchange system. FIG. 2 shows multiplexing. FIG. 3 shows in parts (a) through (e) an example of the bit structures of respective channels.
That is, as shown in FIG. 2, a multiplexer (MPX) 112 multiplexes data Da, Db, Dc and Dd having speeds of 1.2 kbps, 1.2 kbps, 2.4 kbps and 1.2 kbps from terminals 111a, 111b, 111c and 111d, respectively, for transmission as data De having a speed of 64 kbps.
In this case, when data Da through Dd from respective channels are converted "as is" to 64 kbps data only the least significant bit B0 amongst 8-bit data B0 through B7 contains a significant datum, as shown in parts (a) through (d) of FIG. 3.
Therefore, there is a desire to realize a subrate exchange for a bit unit multiplexed transmission over a single channel, which exchanges data Da through Dd from plural low speed terminals, as shown in part (e) of FIG. 3.
Such a subrate exchange is effective in raising the efficiency in line utilization of a digital communications network, namely in improving the usage per line.
FIG. 4 is a block diagram showing the configuration of a conventional ISDN transmission line trunk in a 23+D format. In FIG. 4, the transmission line trunk 1 is connected to an exchange network (NW) 2 for exchanging data from a terminal and a call processor (CPR) 3 for exchanging data e.g. from a terminal. The transmission line trunk 1 comprises a line processor (LPR) 4 for controlling the transmission line trunk 1 pursuant to an instruction from the CPR 3; a D channel terminator 5 for terminating a D channel for transmitting control data; a switch 6 (SW 6) for multiplexing B channel data inputted from the exchange network (NW) 2 with D channel data inputted from the D channel terminator 5; and an ISDN-PRI (primary rate interface) terminator 7 provided between the switch 6 and an ISDN transmission line trunk 9.
FIG. 5 illustrates signals flowing in FIG. 4 during each step of multiplexing twenty-three [23] channels of B channel data with one [1] channel of D channel data in a single frame for transmission to a terminating office and of demultiplexing 24 channels of data sent from an originating office into B channel data and D channel data. In FIGS. 4 and 5, these operations are performed in accordance with the following procedures 1 through 6.
1 On receiving an instruction from the CPR (call processor) 3, the LPR (line processor) 4 has the D channel terminator 5 create a D channel signal (a).
2 The SW 6 (switch 6) combines D channel signal (a) with B channel data signal (b) from the NW 2, thereby creating serial data (c).
3 The ISDN-PRI terminator 7 attaches an F bit to the serial data (c) in conformance with a transmission format, thereby generating transmission serial data (d).
4 The ISDN-PRI terminator 7 transmits the transmission serial data (e) through an ISDN transmission line to the terminating office.
5 In a reverse processing, the ISDN-PRI terminator 7 eliminates the F bit from reception serial data (e) from the originating office, thereby creating serial data (f), which the SW 6 demultiplexes into B channel data (g) and a D channel signal (h).
6 The SW 6 supplies to the NW 2 B channel data thus demultiplexed and to the D channel terminator 5 D channel signal (h). The LPR 4 supplies to the CPR 3 D channel signal (h) processed by the D channel terminator 5.
As described above, when communications are performed in a 23B+D format, the D channel also possess a capability for transmitting a 64 kbps control signal. In the example shown in FIG. 1, even though transmission paths between the head office and the branch offices may necessitate a capability of transmitting a 64 kbps control signal, those between a branch office and its sub-branch offices or those between a sub-branch office and its liaison offices may not, and a transmission speed of 32 kbps or even 16 kbps may suffice for these transmission paths. Hence, there has been an inherent problem that the only four [4] or two [2] bits of the eight [8] bits in a D channel signal actually carry significant control data with the remaining bits unused, which lowers an efficiency of line utilization.